In spite of their many advantages, internal combustion engines emit non-negligible levels of sound when functioning, and it is desirable to reduce said level of sound which many consider disturbing.
In order to decrease the sound emission of an internal combustion engine, the engine is connected to a muffler, or silencer, that dampens the sound emission of the engine to a bearable level, and in some cases, also alters the frequency of the emitted sound.
Traditional mufflers comprise a hollow body having an inlet opening and an outlet opening, an inlet pipe extending into the hollow body through the inlet opening, and an outlet pipe extending out of the hollow body through the outlet opening.
The inlet pipe, which is normally connected to the engine, directs the pressurized combustion gas from the engine into the hollow body where it can expand and cool down before exiting the hollow body through the outlet pipe into the atmosphere.
Because of the temperature of the pressurized combustion gas, which is well in excess of 700° C., the preferred material for mufflers so far has been sheet or cast metal. However, in spite of its good heat resistance, metal has the disadvantage of being very heavy.
There is a general trend in transport vehicle manufacturing that consists in reducing the weight of the vehicles in order to reduce their fuel consumption, and most dramatic weight reductions are achieved by replacing heavy metal parts with lighter composite material.
However, such replacements become problematic when, as in the case of mufflers, the part to be replaced is continuously exposed to high temperatures which limit the choice of lighter alternative materials.
The above problems nothwithstanding, attempts in providing partly polymeric mufflers have been made and are discussed below.
U.S. Pat. No. 6,543,577(B1) describes a muffler comprising a polymeric casing made of a polyamide resin filled with glass. However, the inclusion of glass into the polyamide is not enough to give the polymeric casing the desired heat resistance, and so, it is necessary to stuff the cavity formed by the polymeric casing with loose glass wool to arrive at a functioning muffler. Adding the glass wool to the cavity requires an additional step in manufacturing and is therefore not desirable, and furthermore requires either vast quantities to fill up the cavity and thereby prevent the moving of the glass wool within the cavity, or a means of fixation to prevent the glass wool from moving within the cavity.
U.S. Pat. No. 6,668,972(B2) describes a bumper/muffler assembly having an outer shell formed by a composite material, a perforated pipe and a fibrous material such as for example a glass wool felt placed between the pipe and the shell. Again, the placement and eventual fixation of the fibrous material required additional manufacturing steps that are not desirable.
U.S. Pat. No. 7,325,652(B2) describes a muffler/exhaust pipe system having an outer shell formed by a composite material and a perforated pipe extending through the shell, and having a fibrous material, such as a needle felt, placed between the pipe and the shell.
There is therefore a need to provide for a muffler, or silencer, that is lighter than existing mufflers and which can be manufactured in a more efficient manner, while at the same providing excellent heat stability properties.